1. Field of the Invention
The present invention relates to the field of radio frequency identification (RFID) and, more particularly, to a method for adjusting a transmission time of a continuous wave by a reader and the reader for implementing the method.
2. Description of the Related Art
In recent years, RFID technology has attracted more and more attention. Since hundreds of objects can be identified instantly by RFID technology, RFID technology can be applied to various application scenarios, such as logistics or manufacturing. However, in whatever application scenario, processing efficiency is always one of the important criteria for evaluating the performance of an RFID system.
According to the provisions of the protocol, EPC Radio-Frequency Identity Protocols Class-1 Generation-2 UHF RFID Protocol for Communications at 860 MHz-960 MHz Version 1.0.9, established by the worldwide EPC organization, EPC global, in an RFID system, when the distance between a tag and a reader is within the reading range of the reader, the reader will initiate a process to identify the identificatin (ID) of the tag. After having successfully identified the ID of the tag, the tag will enter into an open or secure state, and at this moment, the reader may send a corresponding command to the tag which is in the Open/Secure state to instruct it to perform a corresponding operation. For example, the reader can send a write command, a kill command, a lock command, a block-write command or a block-erase command to the tag which is in the Open/Secure state to instruct it to perform the corresponding write, kill, lock, block-write or block-erase operation. After having received one of the above-mentioned commands from the reader, the tag in the Open/Secure state will first perform a cyclic redundancy check (CRC) to the received command, and if the check is successful, then the tag will perform the operation corresponding to the received instruction and feeds back a response to the reader. If the check fails, however, then the tag will discard the received command and wait for a new command from the reader. Correspondingly, after having sent a command, the reader will further send a continuous wave (CW) to the tag in the Open/Secure state to provide energy needed by the tag to perform the corresponding operation and at the same time wait for a response from the tag. According to the provisions of the protocol, the longest time for a reader to wait for a response from a tag is 20 millisecond (ms). That is, if the reader receives a response from the tag within 20 ms after it has sent a command, then the reader stops sending the CW after having received the response from the tag. On the other hand, if the reader does not receive a response from the tag within 20 ms after it has sent the command, then the reader stops sending the CW immediately. After having stopped sending the CW, the reader may go on to send a new command to the tag in the Open/Secure state. For the sake of ease of description, the time period from the moment when the reader sends out a command to the moment when the reader receives a response from the tag can be referred to as the tag's response time TREPLY. It can be found by statistics that, if both the reader and the tag operate at the highest transmission rate, the TREPLY is approximately 1.5 ms, which is far less than 20 ms. Even if the reader and the tag both have the lowest transmission rate, the TREPLY is approximately 7 ms, which is still far less than 20 ms.
It can be seen from the above-mentioned performance process that, after having sent out a command, if the reader does not receive a response from the tag, then it needs to wait for a period of 20 ms to then send out a new instruction. That is, after having sent out a command, the reader needs to send a CW up to 20 ms. Furthermore, in an RFID system, situations where a reader cannot correctly receive a response from a tag are still quite common. Since the response time of a tag is far less than 20 ms in normal situations, it causes a reader to wait for too long when it cannot receive a response from the tag, which seriously affects the processing efficiency of the RFID system.